Titanium, Ti, atomic number 22, atomic mass 47.9, is a member of group 4 of the periodic table. Pure titanium is a silvery white, ductile metal, melting point (1668.+-.50.degree. C.), boiling point (3500.degree. C.). .alpha.-Ti has hexagonal structure of the magnesium type. It transforms into the body-centered cubic .beta.-phase at 882.5.degree. C. The metal has 4.5 g/cm.sup.3 density and 100-110 Gpa modulus of elasticity at 25 .degree. C. The standard electrode potential of the reaction TIT.sup.2+ +2e is -1.75 V. The corrosion resistance of titanium metal is due to the formation of a thin, dense, stable, adherent surface film of its oxide, which immediately reforms after mechanical damage if oxygen is present in the surrounding medium.
Titanium is widely distributed and is ninth in order of abundance. The most important minerals are anatase (TiO.sub.2), ilmenite (FeTiO.sub.3) perovskite (CaTiO.sub.3), rutile (TiO.sub.2), and sphene [CaTi (SiO.sub.4)O].
The most important and useful mineral for the extraction of titanium and titanium compounds is rutile. Naturally occurring enriched rutile is brown to black in color, and contains 90-97% TiO.sub.2. Proven world reserves of rutile and ilmenite calculated as TiO.sub.2 content have been estimated at 423.times.10.sup.6 to 600.times.10.sup.6 tons. The largest reserves of ilmenite are in South-Africa, India, the United States, Canada, Norway, Australia, Ukraine, Russia, and Kazakhstan, and of rutile in Brazil.
Titanium metal is exclusively produced according to the current Kroll process by reduction of titanium tetrachloride, which is manufactured from natural rutile and from so-called synthetic rutile, obtained from ilmenite.
In the Kroll process, titanium tetrachloride is reduced to titanium metal by either liquid magnesium or sodium in an inert atmosphere of helium or argon at a temperature between 712 and 920.degree. C. The following reactions possibly take place during reduction: EQU TiCl.sub.4 +2MgTi+2MgCl.sub.2 +Mg excess [1] EQU TiCl.sub.4 +4NaTi+4NaCl [2]
The magnesium reduction is carried out in a cylindrically shaped steel vessel, often with a rounded bottom. The surface oxide of the vessel is removed before its use by filling it with hydrogen and heating to above red heat, then flushing out the hydrogen with helium or argon. The reaction vessel sizes range from small units, 3 feet high by 2.3 feet in diameter, producing 250 lbs, to large units, 9 feet high by 5 feet in diameter, producing 3000 lbs of metal.
Magnesium bars for the reduction, with 15 to 20% excess over the theoretical amount, are cleaned, pickled in hydrochloric acid, dried, and put in the reaction vessel.
A shallow dome-shaped lid with two inlet pipes is welded to the open top of the vessel. The vessel is placed in a gas or oil-fired furnace and heated to approximately 850.degree. C. Liquid TiCl.sub.4 is added at a controlled rate through one of the inlet pipes and reacts with molten magnesium to produce titanium and magnesium chloride. The reaction is exothermic and the reactor temperature is maintained between 712.degree. C. and 920.degree. C. by controlling the rate of TiCl.sub.4 addition. The minimum lower temperature of 712.degree. C. is the melting point of MgCl.sub.2. At any lower temperature solidified magnesium chloride is reported to interfere with the reduction reaction. The maximum temperature of 920.degree. C. is restricted because titanium forms an alloy with the steel reactor above 1000.degree. C. and the exothermic reaction becomes too rapid and uncontrollable above 1100.degree. C.
The temperature is maintained at about 850.degree. C. during most of the reduction time, with the exception of the last hour, when it is raised to about 920.degree. C. The second inlet pipe through the lid is used to maintain the pressure of the inert gas in the reactor slightly above atmosphere during the entire reduction process. A 3 feet by 2.5 feet diameter vessel which is charged with 330 lbs of magnesium and 1070 lbs of liquid TiCl.sub.4 will produce 250 lbs of titanium metal over a period of 6 hours.
TiCl.sub.4 reacts completely and 80-85% of the magnesium is used. Attempts to add additional TiCl.sub.4 to react with the excess magnesium have not been successful, as it has been found that some of the TiCl.sub.4 reacts with the titanium instead, to give titanium trichloride and titanium dichloride: EQU 3TiCl.sub.4 +Ti.fwdarw.4TiCl.sub.3 [ 3] EQU 2TiCl.sub.3 +Ti.fwdarw.3TiCl.sub.2 [ 4]
At the completion of the reduction, the rector is removed from the furnace and allowed to cool to room temperature, still under a small positive pressure of inert gas. The weld on the lid is then ground off, the lid removed, and the reactor tipped on its side. In this position, the cool, solidified charge is either bored out on a large lathe or chipped out with a pneumatic hammer, with the product falling into a rolls crusher where it is broken to 1/4 to 1 inch size pieces. The broken mixture of titanium, magnesium chloride and excess magnesium passes through a rotary leacher where dilute hydrochloric acid dissolves the magnesium chloride and magnesium, leaving the titanium metal, which is in the form of sponge. The metal is water washed, dried and compressed into elongated briquettes. The briquettes are welded together to form an electrode which is melted in a vacuum arc-melting consumable electrode electric furnace to produce a homogeneous metal ingot.
There is another alternate method of treating the reactant products. In this method, a tap hole at the bottom of the reactor is opened twice during the reduction and once at the end to draw off most of the magnesium chloride that is being produced. The tap hole is sealed each time by localized cooling with a water jacket freezing a plug of magnesium chloride in the tap hole. The remaining reactant products are cooled to room temperature and removed as before by boring or chipping, after which they are placed in stainless steel baskets and inserted into a vacuum distillation retort, which can be evacuated to 1 mm Hg pressure by a mechanical pump. Distillation is carried out at 875 to 920.degree. C. for 35 hours. The retort is cooled, still under vacuum, to 815.degree. C. and then further cooled to room temperature under an inert gas. Finally, dry air is sucked through for 6 hours to remove any volatiles, giving a total treatment time of about 50 hours.
Titanium sponge is left in the basket after this treatment and is broken loose with a compressed air hammer to be briquetted and vacuum melted as before. Both the remaining magnesium chloride and excess magnesium are distilled off.
In some other instances the vacuum distillation is done directly in the retort, without removing the solidified charge, and here again the magnesium and magnesium chloride are distilled off, leaving titanium sponge to be broken out, briquetted, and vacuum melted.
The titanium metal produced by any of these methods will have a purity of about 99%, with the major impurities being magnesium, iron, chlorine, and manganese.
The reduction of titanium tetrachloride with sodium is in general very similar to that done with magnesium. Purified liquid sodium at a temperature a bit above its melting point of 97.degree. C. is run into the argon-filled reactor and heated to 700.degree. C. Liquid titanium tetrachloride is then dripped in at a rate to maintain the operating temperature between 850 and 900.degree. C. This temperature range is being governed on the low side by the 801.degree. C. melting point of sodium chloride and on the high side by the 1000.degree. C. temperature where alloying takes place between titanium and the steel retort. At the end, the reaction vessel is sometimes first drained of the molten sodium chloride, and in other instances the whole charge is solidified before being bored or chipped out. Sodium chloride is removed from the titanium sponge by leaching in dilute hypochloric or 2% nitric acid. After this, the sponge is water washed, dried, briquetted, and melted in a vacuum to homogeneous metal.
The major difference in the magnesium and sodium processes is that while an excess of magnesium was used, a deficiency of sodium is put in the reactor along with a small excess of titanium tetrachloride. This is to avoid any excess of sodium at the end of the reaction which would be a fire hazard during the leaching procedure, when the metallic sodium could react with water and burn.
A second method of sodium reduction has accurate charges of molten sodium put into large argon-filled and sealed pots, which are then cooled and moved to the reduction area. In this area, a continuous reactor is charged with accurate amounts of TiCl.sub.4 and metallic sodium, giving a mixture of titanium dichloride and sodium chloride by the reaction: EQU 3TiCl.sub.4 +6Na3TiCl.sub.2 +6NaCl [5]
This produced mixture is then run into the first sodium-charged pots. These pots are now transported to a furnace where at 850 to 900.degree. C. the second reduction stage of titanium dichloride with sodium takes place, to produce titanium sponge and sodium chloride: EQU TiCl.sub.2 +2NaTi+2NaCl [6]
The red-hot pots are removed from the furnace to cool after the reaction is completed. Then, as before, the solidified product is chipped out, crushed, and leached in dilute HCI to dissolve out the sodium chloride; and after water washing, the titanium sponge is dried, briquetted, and vacuum melted to solid ingot.
Both reduction processes are batch operations. They appear too inefficient time-wise. They use TiCl.sub.2 which is costly and requires special care in all operations, such as transferring, storing, and even in utilization. The processes involve too many steps before the final product is produced. They involve heating and cooling cycles which make them energy-wise inefficient.